J. fherm. Biol. Vol. 23, No. I. pp. 59-61, 1998 0 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0306-4565198 $19.00 + 0.00 Pergamon PII: SO306-4565(97)00047-8 THERMONEUTRAL ZONE AND CRITICAL TEMPERATURES OF HORSES KARIN MORGAN Swedish University of Agricultural Sciences, Department of Equine S-750 07 Uppsala, Sweden (Received 22 Februmy~ 1997; accepted in rwised,form Studies, P.O. Box 7046, 25 October 1997) Abstract-l. This short communication discusses the critical temperature and thermoneutral zone data for horses from the three earlier papers (Morgan, 1997a, 1997b; Morgan et al., 1997). Some practical aspects of climatory physiology of horses are also discussed. 2. The baseline rate of total heat loss from horses, calculated by summing the rates of non-evaporative and evaporative heat loss, was 142 W m-‘, and remained stable in ambient temperatures ranging from 5-25°C. 3. The lower critical temperature was found to be 5’C. The upper critical temperature was proven to depend on what definition was chosen. 0 1998 Elsevier Science Ltd. All rights reserved Key Word Index: Horse; critical temperature; thermoneutral CRITICAL TEMPERATURES AND THERMONEUTRAL ZONE In Fig. 1 the obtained &non-evaporative heat loss; with the theoretical of an animal’s model (Mount, According total is due to a short-term in evaporative and horses 1973) according with seems more the insulance as the heat production 1979)] minus the minimal Within the thermoneutral thermal insulance [plane of nutrition evaporative heat loss pedagogical ambient metabolic loss. that the true of heat loss and heat balance than this theoretical model, to the heat balance of animals. to Mount is defined shows an increase the non-evaporative limit (1973) the lower critical as “below this limit the rate must rise if deep-body to be maintained’. temperature In Fig. l(a) the total is heat loss below 5°C and this temperature will be the lower critical temperature. This agrees with Young and Coote (1973) who found the lower critical temperature in young horses and indoor horses at and below the lower critical the total thermal insulance is maximized and the non-evaporative inverse (here 2.78 W m increase to high temperatures. one can conclude tool for introduction According temperature (Mount, heat loss. This plateau can not be seen in the the overall result of non-evaporative heat loss, but can be seen in some individual data. In ambient air temperatures temperature, model. This LOWER CRITICAL TEMPERATURE zone the animal adjusts the to regulate complex and homeothermy ambient heat In since the solid lines differ from the dotted lines. Even model can be used as a so, this theoretical temperature decreases. The non-evaporative heat loss reaches a steady state level, which corresponds to the total temperature. effect with a marked were not acclimatized Viewing Fig. l(a-c) model, where it differs, when body heat loss [Fig. l(a) and (c)l, since the of the pattern at 0 W m-l thermal the above 25°C the results of the total heat loss are a lot higher than the theoretical loss; temperature is equal to the body temperature and increases with the inverse (here 7 W m - ’ “C _ ‘) of the minimal maintain nature to theory the non-evaporative starts to temperatures c-vaporative The results are presented a solid line and the theoretical with a dotted line. l(b)] and heat heat loss and heat balance to a thermodiagram. [Fig. (a-total et al., 1997 are presented heat loss) from Morgan compared results order zone; thermoneutrality heat loss increases with the 2 “C _ ‘) of the maximal total to be O-5% UPPER CRITICAL TEMPERATURE thermal insulance. Accordingly, the total heat production [Fig. l(a)] will show an equal increase in The upper critical temperature might be defined in three different ways, according to Mount (1973). The 59 K. Morgan 60 a: Total heat loss the construction n” 500 k 400 of ?? upper whether or not and which physiological production. -5 0 5 10 20 15 25 30 35 40 minimal, (1987) !piii -5 0 . . . . . . . . . . . . . . heat loss . . . . . . 10 15 20 25 30 35 of Worstell Brody (1953) occurs before has reached value, so ct d. McBride (1983) 5 10 15 20 25 30 35 defined Christopherson defined thermoneutral the in which show relationship total; (b) non-evaporative; and (c) evaporative ambient The dotted between (a) heat loss and lines represent model [according to a thermodiagram the solid lines and (Morgan data points the by Mount are results (‘I t/l.. 1997). in the and of heat heat primarily short term transfer production, of and postural present range maintains with of body little or no In the TNZ physical occur in the these responses blood flow changes. definition of Christopherson general thermoneutral zone (1986) as the an animal variation piloerection, Young zone energy expenditure. processes balance air temperature. the thermoneutral - I5 C and 10 C in still air conditions. additional 40 Ambient air temperature [OC] The three diagrams ZONE lone (TNZ) as related to minimal metabolic rate and concluded that the thermoneutral zone for the horses temperature and the minimum THERMONEUTRAL temperatures (1973)] show that insulance is and in the study was between theoretical temperature that the onset of sweating 40 c: Evaporative heat loss Fig. I. on e.g. heat occur together. 5 0 or rectal of that control of heat loss by sweating and vasodilation . Ambient air temperature [“Cl -5 depending one chooses, rate to to be a problem which agrees with an analysis by McArthur of the data tissue resistance . . . Estimates according will differ parameter respiratory confirming I.. vary (Webster. 1991). The present results sweating occurs before the tissue thermal Ambient air temperature [“Cl b: Non-evaporative definition. temperature heat stress is considered productivity f of an absolute critical body to the Based to being skin, on the and Young (1986) a for the horses in the study can be estimated. from the range of steady state level of rate of total heat loss in Morgan ct cd. (1997) to range from 5-25 C. However, upper critical temperature ?? the temperature is defined as the ambient when: metabolic rate increases: the results: is considered to follow the total heat loss (Morgan PI u/., 1997); 2. 20 C when the evaporative heat loss increases (Morgan et rtl., 1997): and 3. 30 ‘C when peripheral (i.e. minimal thermal (Morgan, 1997b). from this estimated deviations temperature will differ when viewing rate individual vasodilation insulance of is maximal the tissue) upper that it is hard to establish critical temperatures show an uniform definition for the upper critical temperature. Webster (1991) argued that the upper critical temperature is not amenable to this will will have thermoneu- also The reflects a span of zone was 25°C in et ml. (1983) and 2OC in the study. According to Cena and Clark (1979) most homeotherms have a narrow thermoneutral zone, and wider temperature ranges and lower critical temperature are associated with both greater insulation and size. The rather wide range in thermoneutral zone of horses is associated a fairly well insulated with being large homeotherm. PRACTICAL These different 1997a) 5-25 C. in the thermoneutral the study of McBride that that the steady state level of the heart rate (Morgan, thermoneutrdl zone of evaporative heat loss increases; or ?? the tissue thermal insulance is minimal I. 25 C if the metabolic for the horses, tral zone. It is noticeable ?? the These three temperatures be a generalization ASPECTS The thermoneutral zone, the non-evaporative and evaporative heat loss, together with the design and construction of the building will affect the Thermoneutral zone and critical temperatures of horses microclimate in a stable. The ambient air temperature in the stable should be regulated to fall within the thermoneutral zone so as to be thermally neutral for the horse. During the winter season the ventilation rate is regulated to remove moisture and carbon dioxide. The non-evaporative heat loss from the horse in relation to heat loss through the building fabric and in the exhaust air is used to calculate the need for additional heating. The optimal set-point for the ambient air temperature in the stable during the cold season is just above the lower critical temperature of the horse. The evaporative heat loss will then be minimal and the non-evaporative heat loss will contribute positively to the heat balance of the stable. Ventilation rate and extra heating will be minimized, which saves money. In the warm season, the main purpose of ventilation will be to limit the temperature increase in the stable in relation to the outside ambient air temperature. Altered insulation by shearing and covering was studied (Morgan, 1997a). In the cold season it can be of advantage to shear horses that are exercised and trained for competitions, since according to Cena and Clark (1979) hairlessness is an advantage in facilitating sweating, which can be of importance for the exercising horse. Also, the managing of the horse is facilitated and one might speculate on a faster recovery after exercise. When the ventilation rates are controlled to exhaust moisture and carbon dioxide, a decreased evaporative heat loss like in a sheared horse is an advantage, since the ventilation rates can be keep down. The increased non-evaporative heat loss in a sheared horse will be favourable for the climate, since heat from the horses will keep the indoor temperature that a sheared feeding up. It is important to point out horse needs a rug or complementary to ensure that it can maintain body core temperature. SUMMARY The thermoneutral zone, defined as the range of temperatures in which an animal maintains body temperature in the short term with little or no additional energy expenditure, was estimated in general for these horses to range from 5-25°C. It was shown that the upper critical temperature is hard to 61 define and varied between 20°C 25°C and 30°C depending on definition. From the discussion on upper critical temperature, it was concluded that onset of sweating occurred before tissue insulance had reached the minimum value, so that control of heat loss by sweating and vasodilation occurred together. REFERENCES Cena, K. and Clark, J. A. (1979) Transfer of heat through animal coats and clothing. In Inrernational Review of Physiology. Environmenral Physiology III, ed. D. Rohertshaw, Vol. 20, pp. 142. University Park Press, Baltimore, USA. Christopherson, R. J. and Young, B. A. (1986) Effect of cold environments on domestic animals. In Grazing Research at Northern Latitudes, ed. 0. 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